A wind park generally comprises a plurality of wind turbines to generate output power that is usually fed into a grid for distribution to one or more population centers or industrial regions. Because of the fluctuating nature of the energy source, namely the wind, the output of each wind turbine can fluctuate, and the output of the wind park as a whole will also fluctuate.
However, to ensure that a wind park may feed into a grid, the wind park must comply with certain grid codes at the point of common coupling or point of common connection (PCC). The PCC is the point at which the local regulation or grid code requirements should be satisfied. Electric properties at the point of common connection may be directly or indirectly controlled by a park controller (also referred to as “park pilot”, “wind farm controller”, “high-performance park pilot (HPPP)”) to satisfy particular requirements.
Different countries or regions can have different grid code requirements. To this end, the park controller monitors the output of the wind park and generates (or “dispatches”) setpoints to the controllers of the wind turbines on the basis of the monitored output. As “setpoint” is to be understood a value or set of values that is used as a reference by a wind turbine controller. The park pilot can therefore monitor the overall performance of the wind park and manage the wind turbine controllers accordingly.
In the known control systems, a grid code specifies requirements for various operating parameters such as allowed operating power factor range, allowed operating voltage range, etc. For example, a grid code might require that the power factor should be within a range of 0.95 inductive and 0.95 capacitive and that the voltage at PCC point remain within 0.95 and 1.05 of the nominal voltage. During operation, the prior art control system monitors the output of the wind park and adjusts a setpoint which will be dispatched to wind turbines of the wind farm on the basis of a main control mode. A limit controller or “limitation controller” will generate a limit set consisting of an upper bound limit and a lower bound limit, and will forward this to the main controller.
The setpoint generated by the main controller will be constrained by the upper and lower bound limits delivered by the limit controller. For example, a known type of park pilot can operate in only one of two modes at any one time—a voltage limitation mode or a power factor limitation mode, and therefore comprises a voltage limitation controller and a power factor limitation controller. When working in voltage limitation mode, the park pilot observes the output voltage at the PCC, determines whether or not this is within the specified range, and the voltage limitation controller will generate a limit set which will be used as a constraint for the setpoint generated by the main controller. The resulting setpoint is sent to the wind turbines of the wind park. At some point, the park pilot changes over into power factor limitation mode, monitors the power factor, determines whether or not this is within the specified range, and the power factor limitation controller will provide a new limit set and use this as a constraint for the setpoints of the its main controller. The resulting setpoint is issued or dispatched to the wind turbines of the wind park. Characteristic of this known type of control system is that the control system can only work in one mode at any one time, and only one aspect of the grid requirement can be considered while the park pilot operates in that mode. Therefore, a known control system can fail to simultaneously fulfill multiple grid code requirements. In the example given above, the setpoint generated during voltage limitation mode may result in a power factor that does not comply with the power factor grid code requirement. In the same way, the setpoint generated during power factor limitation mode may result in a voltage that does not comply with the voltage grid code requirement.